In deep sea oil or gas production sites, subsea power grids are installed for providing the required electrical power for electrical actuators like electric motors and electric pumps. For controlling an electric motor, voltage, current and AC frequency may be varied. To this end, a variable speed drive (VSD) may be provided in the power grid. These variable speed drives contain inter alia DC link capacitors.
All components including the capacitor inside the subsea VSD enclosure will be located in a pressure compensated fluid volume, e.g. a dielectric fluid such as Midel 7131. This means that all the electrical components will be in contact with the dielectric fluid and will be exposed to the same ambient pressure as the water pressure surrounding the variable speed drive. The current design depth of the subsea variable speed drive is 1 to 3000 meters, which gives a surrounding seawater pressure of approximately to 300 bar resulting in a fluid pressure inside the variable speed drive of approximately 1 to 300 bar. Applications at even higher water depths up to 5000 meters are foreseeable.
Since the DC link capacitor will be exposed to the ambient pressure at the seabed, it must be designed to withstand the mechanical stress that is caused by this pressure. The capacitors used for frequency conversion are often implemented as film capacitors, e.g. metalized film capacitors. The film material normally used in such a film capacitor may be polypropylene, but is not limited to this material wherein the thickness can vary between 1 μm and several hundreds of μm. Before the films are wound in many turns, the films, plastic films, are metalized with a thin layer of aluminium, zinc or other metals in order to provide the metal layer between the dielectric layers. Different methods for winding a metalized film capacitor are known. One possibility to generate such a metalized film capacitor is the stacked winding method, in which one or more films are wound in many turns around a polygon shaped core. The flat sections of the winding are cut out and are used as capacitors while the corner/bent sections are discarded. Another method is a flat winding, in which one or more films are wound in many turns around the core which is often cylindrical. When the winding is completed, the core is removed and the winding is stamped to a flat shape using a strong force.
After winding is completed, the winding is put onto a machine where a metal spray (metal vapour) is applied to both side surfaces of the winding to form contact layers which build electrical terminals. As metals, zinc, aluminium, or zinc and aluminium mixtures may be used, however, also other metals may be used. Furthermore, an additional outer layer of tin spray may be applied to improve the solderability of wires/contact elements to the contact layers.
It is known that an interlayer pressure, i.e. a pressure distributed over the entire surface of the capacitor, between each layer in the capacitor and which is not to be confused with the ambient pressure, is necessary for metalized film capacitors to function when the ambient pressure increases. In atmospheric pressure it is relatively easy to obtain a sufficient interlayer pressure, since the shape of the capacitor is constant. However, due to the volume compressibility of the dielectric film material, the capacitor will be compressed and deformed as the ambient pressure increases. By way of example, a typical change in height of a capacitor stack that is 400 mm high can be around 10 mm in subsea application, resulting in a lower height of the capacitor stack of 390 mm.
One aspect of a metalized film capacitor is its ability to self heal. If a local breakdown occurs in the dielectric inside the capacitor, an arc will form and the metal layer around the fault will evaporate. Eventually the arc will be quenched and since the metal layer is gone, the faulty point will be isolated from the rest of the capacitor and the voltage withstand strength of the capacitor will be restored. It is known that increased interlayer pressure reduces energy that is consumed in one of the self healing events. A reduced energy means less heating of the film near a faulty area and a smaller possibility of another breakdown due to excess heating. As a consequence this means that the lifetime will be improved when sufficient interlayer pressure is maintained. Test results in simulations have furthermore shown that a high ambient pressure will significantly impact the capacitor. Due to the large difference of the material properties in the dielectric film, e.g. polypropylene, and the metal spray of the contact surfaces at opposite ends of the capacitor, the capacitor will be deformed when the ambient pressure increases. This deformation will result in sections of the dielectric film having interlayer pressure that is below a needed interlayer pressure.